general questions to get started to build a reponsive theme!!

hello,
I am trying to learn building themes , so I learned little htm,css,php .
now I am trying to make my own theme , but I want to have it also responsive.
I am going to follow also courses on linda.com.
now to make things more fast I liket to have a responsive free theme as a guidelines for me . I see some responsive free themes which are made by bootsrap .
so my question is now:
1- if I want to use this kind of themes which are made with bootsrap I thing I have to install bootsrap framework am I right or wrong?
2- I thing also then I have to learn bootsrap or it is not really needed?
3-is bootsrap framework is free or you have to pay some fee?
4-in modern time wat is the best way to create fully responsive themes on your own desires is that bootsrap is a must or also you can do it in usually ways?
very short answers wil be enough.
thanks
johan